Al-Sadr has called for a protest on April 9 in Sadr City against the presence of US forces in Iraq.

His office said it expects at least one million people to turn out for the protest.

Sunday's clashes in Baghdad formed a violent backdrop to political efforts to isolate the Mahdi Army.

All the major Shia, Sunni and Kurdish parties have closed ranks to pressure al-Sadr into disbanding the Mahdi Army or be barred from political life, according to legislators and officials involved in the effort.

They said a first step would be to add language to a draft election bill banning parties that operate militias from fielding candidates in provincial balloting this autumn.

Al-Sadr controls 30 of the 275 parliament seats, a substantial figure but not enough to block legislation.

Mosul kidnapping

In northern Iraq, meanwhile, Iraq's security forces freed 42 university students who had been kidnapped by armed men on Sunday, a local army commander said.

Brigadier-General Khalif Abdul-Sattar said the students were waylaid about 30km south of Mosul on the main highway to Baghdad.

Three other students on a second bus were injured when armed men opened fire as the driver managed to speed away, he said.

It was not immediately clear where the students were coming from or where they were going.